The invention relates to a process for preparing water-based pyrotechnic active compositions containing metal powder, to coated metal powders and to the use thereof.
Protechnic active compositions frequently contain, as the active principle, red phosphors in combination with metal powder, in particular with aluminium or magnesia. Hitherto the preparation of such active compositions was carried out by dissolving a binder in a chlorinated hydrocarbon, suspending the magnesium powder or aluminium powder and the red phosphorus in this solution and granulating the suspension by evaporation of the solvent. These granules could then be readily further processed and metered. For reasons of protection of the environment, however, chlorinated hydrocarbons cause problems, and their use will be restricted in the course of the next few years, so that they must be replaced by other solvents. Processing in an aqueous system, which is the most easily handled with respect to safety, emission problems and toxicity problems, would of course be advantageous. Metal powders cannot, however, be readily suspended in water, since this might lead to an explosive reaction with the formation of hydrogen and hydroxides. Moreover, they are partially inactivated by the formation of hydroxides. The metal powder, such as magnesium powder or aluminium powder, must therefore be pretreated in such a way that it cannot react with water.
It is already known to modify metal powders by chemical oxidation or physical processes in such a way that no harmful reactions occur on contact with water. Thus, for example AT-B 236,729 and AT-B 240,128 have disclosed processes for chemically oxidizing aluminium powder and magnesium powder, wherein the granules forming the powder are coated with an oxide skin which protects the metal. It is also known to provide metal powders with a coating, for example of stearic acid. A disadvantage of these processes is, however, that either no adequate protection against water is obtained or that the reactivity is diminished to such an extent that the metal powders can no longer satisfactorily undergo the desired reaction, or not at all.
From DE-A 3,626,861, a process was known for preparing propellant powder, which is safe to handle and is based on crystalline explosives, wherein the individual crystals of the explosive were enveloped by a resin in a fluidized-bed process. Moreover, U.S. Pat. No. 3,706,611 has disclosed a process for preparing a pyrotechnic plastic composition which consists of a liquid polysulphide polymer, a rubber-forming agent, a metal powder, an organic oxidizing agent and a dye, the metal powder and the liquid polymer being first mixed at very low pressure and the oxidizing agent and dyes then being added gradually.